Problem 35 - Chemistry The Central Science In Si Units

Question

Label each of the following substances as an acid, base, salt, or none of the above. Indicate whether the substance exists in aqueous solution entirely in molecular form, entirely as ions, or as a mixture of molecules and ions. (a) HF, (b) acetonitrile, \(\mathrm{CH}_{3} \mathrm{CN},(\mathbf{c}) \mathrm{NaClO}_{4},(\mathbf{d}) \mathrm{Ba}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

(a) HF - Acid, mixture of molecules and ions. (b) CH₃CN - None, molecular form. (c) NaClO₄ - Salt, entirely ions. (d) Ba(OH)₂ - Base, entirely ions.

Step 1: Analyze the Substance (a) HF

**HF (Hydrofluoric Acid)** is categorized as an acid, specifically a weak acid. In aqueous solution, it exists as a mixture of molecules and ions because it partially ionizes in water: \[ \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \] Thus, some HF molecules remain undissociated while others dissociate to form ions.

Step 2: Analyze the Substance (b) CH₃CN

**CH₃CN (Acetonitrile)** is an organic compound and neither an acid nor a base (at least not in the context of typical acid-base behavior in water), nor a salt. It exists entirely in molecular form in aqueous solution as it does not ionize: \[ \text{CH}_3\text{CN} \rightarrow \text{CH}_3\text{CN}_{(aq)} \] Therefore, no ions are present.

Step 3: Analyze the Substance (c) NaClO₄

**NaClO₄ (Sodium Perchlorate)** is a salt. In aqueous solution, it dissociates completely into its ions: \[ \text{NaClO}_4 \rightarrow \text{Na}^+ + \text{ClO}_4^- \] Thus, it exists entirely as ions in water.

Step 4: Analyze the Substance (d) Ba(OH)₂

**Ba(OH)₂ (Barium Hydroxide)** is considered a base. In aqueous solution, it dissociates completely into ions: \[ \text{Ba(OH)}_2 \rightarrow \text{Ba}^{2+} + 2\text{OH}^- \] Therefore, it exists entirely as ions in water.

Key Concepts

Aqueous Solutions

Aqueous solutions are a central concept in acid-base chemistry. When a substance is dissolved in water, it forms what's called an aqueous solution. The solute, which is the substance being dissolved, can behave in various ways depending on its chemical nature. Some substances completely dissociate into ions, while others may not dissociate at all and remain in their molecular form.
This behavior is critical because it determines the role a substance plays in reactions. For instance:

• Acids might partially or fully dissociate to release hydrogen ions ( H^+ ), which are crucial in determining acidity.
• Bases often dissociate to release hydroxide ions ( OH^- ).
• Salts generally dissociate into their constituent ions.

In our dog-eared chemistry textbooks, we often encounter examples like NaClO₄ dissolving in water, which results in a solution where it entirely separates into Na⁺ and ClO₄⁻ ions, typical behavior for salts.

Molecular Form

Substances in aqueous solutions can sometimes exist entirely in their molecular form. This means the molecules don't dissociate into ions. Rather, they stay intact as whole molecules dissolved in water. This typically occurs with certain non-electrolytes, which do not produce any charged particles when in solution.
Acetonitrile, or CH_3CN , is a prime example of this. When dissolved in water, it doesn't release ions but stays as whole molecules of CH_3CN . This scenario is common in organic compounds that do not readily ionize, making them distinct from acids, bases, and salts which typically do form ions.

Ions

Ions are electrically charged particles that form when a substance dissociates in an aqueous solution. They are central to how reactions occur in water, influencing everything from conductivity to reactivity.
There are different kinds of ions:

• Cations, which have a positive charge, such as Na^+ and Ba^{2+}.
• Anions, which have a negative charge, such as ClO_4^- and F^-.

When substances like NaClO₄ and Ba(OH)₂ are dissolved in water, they fully break down to form these ions. Such ionization is what's responsible for their ability to conduct electricity and participate in chemical reactions typical of ionic compounds.